Piggy back method for producing ceramic fibers and non-circular ceramic fibers in particular

ABSTRACT

Special C-shaped carbon fibers, melt spun from mesophase pitch, were used as micro-molds to form nested dual fibers and ceramic fibers. By wetting these carbon fibers in a wet chemical precursor, and subsequently heat treating, ceramic fibers of various compositions were formed. Also, through proper control, carbon-ceramic nested fibers were produced. The ceramic materials were silica, alumina, silicon carbide, hydroxyapatite, and zirconia. The ceramic fibers could be formed with non-circular transverse cross-sectional perimeters.

BACKGROUND OF THE INVENTION

The present invention relates to nested ceramic fibers, non-circularceramic fibers, and a piggyback micro-mold method for producing bothkinds of ceramic fibers as well as circular ceramic fibers.

It is desirable to provide ceramic material in a fiber form so that theceramic fibers can be suspended in a binder and more easily processed toform a ceramic/binder composite material. However, the production ofceramic fibers has been expensive. Ceramic refractory fibers ofalumina-silica, alumina and zirconia can be produced by a blowing methodwhich calls for shattering a stream of molten ceramic with a jet of airor steam. Production of ceramic fibers by this method has thedisadvantage of requiring large amounts of capital investment in processand control machinery. Other methods which have been used to produceceramic fibers include spinning methods, continuous filament methods,colloidal evaporation processes, vapor deposition single-crystal method,whisker method, oxidation method, crystallization method andpseudomorphic alteration method.

Many ceramic materials that would be desirable in the form of fibers arenot spinnable. Most ceramic fibers presently made are limited by theprocess of drawing the fibers from a melt and rapidly cooling to preventdevitrification or crystallization. Phase separation can also be aproblem. This is particularly true in the case of glass fibers. Ceramicfibers produced by melt drawing are further limited to compositionscontaining appreciable amounts of so-called "glass forming oxides" suchas SiO₂, B₂ O₅, or P₂ O₅. Melt drawing requires high temperatures andspecial equipment for the drawing of the fibers.

Synthesis of single or multicomponent oxides via a sol-gel process hasbeen possible since at least 1969 when Dislich and Hinz developed achemical basis for the preparation of multicomponent oxides. Theformation of glasses and ceramics via the sol-gel process results invery homogeneous, high purity materials from the mixing that occurs onthe molecular scale. The glasses can be formed using the sol-gel processusing relatively low temperatures. The absence of phase separation andcrystallization during sol-gel processing allows it to be used toproduce glasses and ceramics from compositions that would exhibit phaseseparation and crystallization during conventional melting processes.

However, sol-gel processing does pose a number of difficulties. Forexample, one drawback to sol-gel processing is the difficulty is forminglarge monolithic pieces. Many compositions formed via the sol-gelprocess are limited to powders or thin films.

Thus, the formation of fibers by spinning or drawing from a sol isexpensive and difficult depending on the material to be spun or drawn.

Composite materials including graphite fibers in a matrix can be formed,and composite materials can be formed with ceramic fibers in a matrix asin U.S. Pat. No. 4,454,190. However, ceramic materials are difficult towork with because of their amorphous properties prior to sintering andtheir brittle properties after sintering. Moreover, the production of afiber having a ceramic portion and a non-ceramic portion is not known.

OBJECTS AND SUMMARY OF THE INVENTION

A principal object of the present invention is to provide a dual fibercomposed of ceramic material nested in another material.

It also is a principal object of the present invention to provide amethod of producing a dual fiber having a ceramic portion nested withina portion formed of another material.

Another principal object of the present invention is to provide a methodof producing a ceramic fiber that requires less capital investment inprocessing and control apparatus than conventional methods.

Still another object of the present invention is to provide ceramicfibers having non-circular transverse cross-sectional perimeters.

Yet another object of the present invention is to provide a method ofproducing a ceramic fiber having a non-circular transversecross-sectional perimeter.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and attained bymeans of the instrumentalities and combinations particularly pointed inthe appended claims.

To achieve the objects and in accordance with the purpose of theinvention, as embodied and broadly described herein, a piggyback methodof forming an elongated dual fiber having an elongated ceramic portionand an elongated non-ceramic portion of starting with a wet chemicalprecursor and a non-ceramic fiber having an elongated cavity therein,comprises: wetting the non-ceramic fiber in the wet chemical precursor;setting the wet chemical precursor portion of the wetted non-ceramicfiber; placing the wetted non-ceramic fiber into an oxygen-freeenvironment; heating the set wetted non-ceramic fiber in the oxygen-freeenvironment to a temperature sufficient to sinter or pyrolyze thechemical precursor portion into a ceramic portion.

Preferably the non-ceramic fiber is formed of one of the followingmaterials: any carbon such as pitch, mesophase carbon, polyacrylinitrile(hereafter PAN), or any polymer such as polyethylene, polyvinyl alcohol,nylon, and the like. Preferably, the elongated non-ceramic carrier fiberportion has an elongated cavity with either a C-shape or an annulusshape to its transverse cross-sectional perimeter.

In further accordance with the objects and purpose of the invention, adual fiber comprises an elongated non-ceramic carrier fiber portionhaving a non-circular transverse cross-sectional perimeter over asubstantial length thereof and defining a cavity therein; and anelongated ceramic filler portion nesting in the cavity.

In further accordance with the objects and purpose of the invention, asembodied and broadly described herein, the piggyback method of forming adual fiber can be transformed into a piggyback micro-mold method offorming a ceramic fiber by adding the step of removing the non-ceramicfiber portion of the dual fiber to yield a ceramic fiber. Preferably,the step of removing the non-ceramic fiber portion of the dual fiber iscarried out either by heating the dual ceramic/non-ceramic fiber in anoxygen-containing environment until the non-ceramic fiber portion isburned away or by mechanically stripping away the non-ceramic fiberportion until only the ceramic portion remains.

In still further accordance with the objects and purpose of theinvention, as embodied and broadly described herein, an elongatedceramic fiber has a non-circular transverse cross-sectional perimeterover a substantial length thereof.

The accompanying drawings, which are incorporated herein and constitutea part of this specification, illustrate embodiments of the inventionand, together with the description, serve to explain the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the piggyback micro-mold method of the presentinvention;

FIG. 2 is a schematic of an embodiment of a non-ceramic fiber having anelongated cavity therein;

FIG. 3 is a schematic of another embodiment of a non-ceramic fiberhaving an elongated cavity therein;

FIG. 4 is a SEM photograph at 2000 times magnification showing theresult of performing one of the steps of the method of the presentinvention;

FIG. 5 is a SEM photograph at 200 times magnification showing aplurality of ceramic fiber products of the present invention;

FIG. 6 is a SEM photograph at 800 times magnification showing theceramic fiber product of the present invention having a non-circulartransverse cross-sectional perimeter;

FIG. 7 is a SEM photograph at 240 times magnification of a ceramic fiberproduct of the present invention produced according to the method ofExample 2;

FIG. 8 is a SEM photograph of a ceramic fiber product of the presentinvention produced according to one of the methods of Example 3;

FIGS. 9a and 9b are SEM photographs of ceramic fiber products of thepresent invention produced according to another of the methods ofExample 3;

FIG. 10 is a SEM photograph of a ceramic fiber product of the presentinvention produced according to yet another of the methods of Example 3;

FIGS. 11a and 11b are SEM photographs of ceramic fiber products of thepresent invention produced according to still another of the methods ofExample 3;

FIG. 12 is a SEM photograph of a ceramic fiber product of the presentinvention produced according to one of the methods of Example 4;

FIG. 13 is a SEM photograph of a ceramic fiber product of the presentinvention produced according to another of the methods of Example 4;

FIG. 14 is a SEM photograph at 50 times magnification of ceramic fiberproducts of the present invention produced according to the method ofExample 5;

FIG. 15 is an illustration of the effect of viscosity of the wetchemical precursor on the shape of the nested ceramic portion of thedual fiber of the present invention;

FIG. 16 is a SEM photograph of silica fiber products of the presentinvention produced according to the method of Example 1 with a trilobalnylon micro-mold fiber;

FIG. 17 is a SEM photograph at 450 times magnification of a trilobalnylon fiber micro-mold for use in the method of the present invention;and

FIG. 18 is a SEM photograph of a dual fiber of the present inventionhaving a silica ceramic nested portion and a C-shaped non-ceramicmicro-mold portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings and described in specific examples hereinafter setforth.

In accordance with the present invention, a piggyback micro-mold methodis provided for forming a dual fiber having a substantial ceramicportion. The method of the present invention requires as startingmaterials, a wet chemical precursor, such as a sol-gel, and an elongatednon-ceramic fiber having a non-circular transverse cross-sectionalperimeter over a substantial portion of the length thereof. Preferably,the transverse cross-sectional perimeter of the non-ceramic fiberdefines an elongated cavity therein. These non-circular non-ceramicfibers act as micro-molds for a wet chemical precursor that is preparedfrom materials that would be desirable in the form of ceramic fibers.The materials constituting a part of the wet chemical precursor are notnecessarily spinnable or easily formed into fibers by conventionalmethods.

In accordance with the method of the present invention, an elongatednon-ceramic fiber is provided having a non-circular transversecross-sectional perimeter over a substantial portion of the lengththereof. As embodied herein and shown for example in FIGS. 1 and 2, acarrier micro-mold fiber 20 has a C-shaped transverse cross-sectionalperimeter 22. As shown in FIG. 3, a carrier micro-mold fiber 24 has anannulus-shaped transverse cross-sectional perimeter 26. Fibers 20, 24constitute suitable non-ceramic carrier fibers comprising components ofthe starting materials of the method of the present invention. Becauseof the non-circular transverse cross-sectional perimeters of the fibersused as micro-molds in the method of the present invention, an elongatedcavity 28 is provided in the micro-mold carrier fibers.

Preferably, the non-ceramic carrier micro-mold fiber is formed fromcarbon or an organic polymer. The carbon can be in the form of pitch,meosphase or polyacrylinitrile. The organic polymer can be polyethylene,polyvinyl alcohol, nylon, or the like. Preferably, the polymericmicro-mold carrier fibers are cross-linked. Carbon or graphite carriermicro-mold fibers having a C-shaped or annulus-shaped profile can beobtained using the method set forth in co-pending U.S. patentapplication Ser. No. 820,734, filed on Jan. 21, 1986, which is herebyincorporated herein by reference and as to which, applicant is aco-inventor. According to this method, the carbon micro-mold fibers arepreferably rendered infusible. Polyethylene, nylon, andpolyacrylinitrile carrier micro-mold fibers can be obtained usingconventional spinning procedures for commercial polymers.

Cavity 28 is available for receiving a wet chemical precursor which canbe heat treated to form ceramic material. Examples of wet chemicalprecursors include: sol-gels, slurries, melts, suspensions anddispersions. Examples of ceramic materials used to form the wet chemicalprecursors which constituted one component of the starting material forthe method of the present invention are: silica, alumina, siliconcarbide, zirconia and hydroxyapatite.

A sol is a suspension of fine solid particles in a liquid. A sol isusually the first stage in the formation of a gel, which is a two-phasecolloidal mixture of a liquid and a solid. A slurry defines coarseparticulate matter combined with a liquid. A melt is the liquid phase ofa substance. Suspensions and dispersions are used interchangeably todefine particulate matter on the order of between 1 micron (10⁻³ mm.) toone millimicron (10⁻⁶ mm.) in size suspended in a liquid.

In one particular sol-gel formation technique, an organosilicate such astetraethyl orthosilicate (TEOS) is mixed with a solvent, ethyl alcohol,for example. H₂ O is then added to cause hydrolysis of the Si--OR (R=C₂H₅) groups to form Si--OH. A continuous Si--O--Si network can then formthrough polymerization.

Hydrolysis can occur in the following ways:

    Si(OR).sub.4 +H.sub.2 O (RO).sub.3 --Si--OH+ROH            (1)

    Si(OR).sub.4 +2H.sub.2 O (RO).sub.2 --Si--(OH).sub.2 +2ROH (2)

    Si(OR).sub.4 +3H.sub.2 O (RO)--Si--(OH).sub.3 +3ROH        (3)

    Si(OR).sub.4 +4H.sub.2 O Si--(OH).sub.4 +4ROH              (4)

Equation 4 shows a fully hydrolyzed system, in which all four (OR)groups are replaced by (OH) groups. This process consumes H₂ O,requiring four moles of H₂ O for every mole of TEOS. However, in thepolymerization step the Si--OH can react to form bridging oxygensbetween silicate molecules, releasing H₂ O as shown in equation (5):

    2Si--OH Si--O--Si+H.sub.2 O                                (5)

The H₂ O that is condensed during polymerization can then cause furtherhydrolysis, allowing complete hydrolysis with less than four moles of H₂O for every mole of TEOS.

As polymerization continues, a silicate network is formed withincreasingly large molecules. At some point, the Si--O--Si linkagebetween particles together to form a rigid, porous, tangled network ofbranching chains.

Hydrolysis and condensation reactions are critical steps, since theydetermine whether the subsequent processing steps to form the sol-gelwill be successful. Both hydrolysis and condensation are affected by thefollowing factors: the H₂ O to TEOS molar ratio, the solvent to TEOSratio, temperature, pH, atmosphere, mixing speed, and time.

Hydrolysis and condensation rates of a sol-gel can be increased byaddition of acid or base as a catalytic agent. In acid catalyzed systems(pH less than two), hydrolysis is rapid, but gelation is slow. This isdue to a small amount of linkage between particles, which is a result ofthe small ionic charge of the particles. The ionic charge of theparticles, which increases as pH increases, is needed to facilitateparticle-particle collisions. The result is a tendency towards chainsrather than a three dimensional network.

In base catalyzed systems (pH greater than two) the highly chargedpolymers react with monomers and rapidly decrease the concentration ofmonomers. This causes growth of individual spherical polymeric species.The high surface charge on the spherical particles causes mutualrepulsion, therefore there is little particle-particle reaction, whichleads to a precipitate rather than a gel at very high pH.

In a mixed catalyst system, acid is added initially, followed by addingbase at a later time. By using this method, gelation can occur fasterand at lower water contents than by using only acid or only base. Thisis because under basic conditions, SiO₂ is more soluble than in acid,which facilitates partial depolymerization. Since hydroxide ions attackSi--O--Si bonds, the hydrolysis rate is comparable to the condensationrate. The increase in the hydrolysis rate leads to dense particles withweak particle-particle bonding.

In accordance with the method of the present invention, there isprovided the step of wetting the non-ceramic fiber in a wet chemicalprecursor. As embodied herein and shown for example in FIG. 1, the stepof wetting the non-ceramic fiber in the wet chemical precursorpreferably comprises submerging non-ceramic fibers 20 beneath thesurface of a wet chemical precursor 30 held in a bath 32. This step alsocan be accomplished by dipping, bathing, spraying, or the like.

In further accordance with the method of the present invention, the wetchemical precursor is allowed to set until it nests in the cavity of thenon-ceramic fiber. For example, the non-ceramic fiber is removed fromthe wet chemical precursor bath. As shown in FIG. 1, this can beaccomplished by draining the liquid through a screen 34 to filter thenon-ceramic fibers. As the non-ceramic fibers are removed from the wetchemical precursor bath, surface tension causes the wet chemicalprecursor to adhere to the non-ceramic fiber. Thus, the non-ceramicfiber micro-mold carries with it a substantial amount of the wetchemical precursor in the elongated cavity of the non-ceramic fiber. Ashort time (on the order of five minutes at room temperatures andpressure) after removal from the wet chemical precursor bath, the wetchemical precursor in the cavity has set sufficiently to nest in thecavity of the non-ceramic fiber. This is shown for example in FIG. 4,for a non-ceramic fiber composed of carbon and a silica wet chemicalprecursor.

As stated above, a metal oxide sol-gel is a suitable wet chemicalprecursor for the method of the present invention. One of the factorsthat affects setting, i.e., gelling, time of a sol-gel is the H₂ Ocontent. For example, as the H₂ O to TEOS molar ratio increases, thegelling time will decrease because of the increase in the number ofreactants. The humidity of the atmosphere above a solution can affectthe water content. Because the gelling time is sensitive to watercontent, the gelling time could be affected by the atmosphere.

In further accordance with the present invention, the non-ceramic fiberwith the wet chemical precursor set and nested in the cavity of thefiber, is placed in an oxygen-free environment and heated to atemperature sufficient to pyrolyze or sinter the chemical precursorportion into a ceramic portion. The actual temperature depends on thecomposition of the chemical precursor portion, the preparation techniquefor the chemical precursor and the sintering desired. The sinteringtemperature cannot be so high as to burn away the non-ceramic fiberportion, unless removal of this portion is desired. As embodied hereinand shown for example in FIG. 1, the oxygen-free environment is providedby filling a heating chamber 36 with a non-oxidizing gas such as inertgases like argon, helium or nitrogen. The temperature inside the heatingchamber is then raised in step-wise fashion over time at a rate of about20° C. per minute to a temperature of at least 600° C. for a period of20 to 30 minutes, or a longer or shorter time as necessary to pyrolizeor sinter the chemical precursor portion into a continuous ceramicportion nested in the cavity of the non-ceramic fiber portion. In thisway, a dual fiber is produced. As shown for example in FIG. 18, the dualfiber has a ceramic fiber portion nested in the C-shaped carbon fibermicro-mold portion. The line in the lower right hand corner of FIG. 18represents a length of 100 microns.

In further accordance with the method of the present invention, anadditional processing step is provided to remove the non-ceramic fiberportion of the non-ceramic/ceramic dual fiber in order to yield aceramic fiber. This ceramic fiber may have a circular or non-circulartransverse cross-sectional perimeter, depending on the shape of thecavity of the micro-mold carrier fiber. As embodied herein and shown forexample in FIG. 1, the step of removing the non-ceramic fiber portionpreferably comprises heating the dual fiber in an oxygen-containingenvironment until the non-ceramic fiber portion is burned away and onlythe ceramic portion remains. Preferably, this is accomplished byevacuating the non-oxydizing gas from the heating chamber andsubstituting into the heating chamber an atmosphere of air. Then thetemperature within the heating chamber is raised to a temperaturesufficient to burn away the non-ceramic portion of the dual fiber. Forcarbon micro-mold carrier portions, a temperature of approximately 1100°C. has been used for a period of time which is sufficient to burn awaythe non-ceramic micro-mold fiber portion of the dual fiber. However, forcarbon micro-mold carriers, a temperature of approximately as low as600° C. can be used to burn away the non-ceramic micro-mold fiberportion of the dual fiber.

In an alternative embodiment of the invention, a non-ceramic micro-moldfiber portion is mechanically removed until only the ceramic portionremains. This can be accomplished by using a separating agent oremploying a shaking technique in the presence of an abrasive agent orinstrumentality.

In another alternative embodiment of the method of the presentinvention, the step of heating the non-ceramic fiber portion having theset wet precursor nested in the cavity can be eliminated, and the methodcan proceed directly to the step of removing the non-ceramic fiberportion by firing same in an air atmosphere. For some set chemicalprecursors, the firing in an air atmosphere is sufficient to pyrolyze orsinter the set chemical precursor into a continuous ceramic portion. Theomission of the step of heating in a non-oxidizing environment can beused when the object of the method is to produce a ceramic fiber ratherthan a dual fiber having a ceramic portion nested in a non-ceramicportion. This is because the firing in an air atmosphere oxydizes thenon-ceramic fiber portion and thus removes same.

The materials chosen to demonstrate the effectiveness of the piggybackprocess as a means of forming ceramic fibers were: SiO₂, Al₂ O₃, ZrO₂,SiC, and Hydroxyapatite.

EXAMPLE 1

A silica sol was prepared using tetraethyl orthosilicate (hereafterTEOS), which can be obtained from Fisher Scientific Co., P.O. Box 829,Norcross, Ga. 30091. The silica sol was prepared by adding anhydrousethyl alcohol to TEOS in the ratio of three moles of alcohol to eachmoleof TEOS. The solution was mixed for 15 minutes before addinghydrochloric acid at a concentration that allowed the molecular ratio ofwater to TEOS to be 6.0 : 1.0 and the concentration of HCl to TEOS to be0.01 : 1.0. The HCl solution was added dropwise over a period of 25minutes. The sol was then covered and mixing continued for two hours.The water to TEOS molar ratio of the above sol was 6.0 : 1.0,corresponding to a solution that does not exhibit spinnability.

Carbon fibers having a C-shaped transverse cross-sectional perimeterwere prepared as described in U.S. application Ser. No. 820,734, filedon Jan. 21, 1986, which is hereby incorporated herein by reference.Applicant is a co-inventor of Ser. No. 820,734.

The C-shaped carbon fibers were submerged into the sol and removed fromthe sol before the sol reached the gelling point. No strict guidelinesapplied to the time at which the carbon fibers were dipped in the sol,except that it had to be after the final acid addition, and beforegelling. The viscosity of the solution increases slowly with time aspolymerization takes place. When the viscosity approaches 1 Pa Sec, itbecomes difficult to separate the fibers from the sol and the fibersalso tend to clump together.

The dipped fibers were set by allowing them to dry in atmosphericpressure at room temperature. The set fibers were examined under a lightmicroscope to reveal that the cavity of the "C" was filled with silicagel. A scanning electron microscope (hereafter SEM) photograph showingthe gel nested within the C-shaped cavity is shown in FIG. 4 at amagnification of 2000 times actual size. The set fiber is heated fromroom temperature to 800° C. at a rate of 20° C. per minute tosimultaneously burn off the carbon carrier fiber portion and completethe gel-to-glass transformation. Pure, transparent, amorphous silicafibers were the result and are shown in a SEM photograph in FIG. 5 at amagnification of 200 times. The tensile strength of these fibers was inexcess of 150,000 psi. An individual silica fiber is shown in an SEMphotograph in FIG. 6 at a magnification of 800 times. Note that theceramic fiber shown in FIG. 6 has a non-circular transversecross-sectional perimeter which is shaped like a crescent moon.

Other silica sols were prepared with varying amounts of water, alcohol,and acid. Acid-base catalyzed systems were also used.

EXAMPLE 2

Aluminum-isopropoxide, Al(OC₃ H₇)₃, supplied by Alfa Products, Inc., 152Andover Street, Danvers, Mass. 01923, was used as the starting materialfor preparing an alumina sol. Twenty (20) grams of Aluminum isopropoxidewas added to 180 ml of deionized water which had been placed in a 250 mlbeaker and heated to maintain the water temperature between 80° C. and90° C. The beaker was only partially covered, allowing evaporation ofwater and alcohol from the sol. The resulting slurry was mixedvigorously for 25 minutes before 5.8 ml of 1.2m HCl was added to achievea 0.07 : 1.0 acid to alkoxide molar ratio. During the HCl addition,mixing was continued and the temperature maintained at 80° C. Mixing andtemperature maintenance continued for four hours in a covered container.When the volume of the sol was approximately one-third (1/3) that of thestarting volume (approximately 4 hours later), C-shaped carbon fiberswere submerged in the sol for about 5 minutes. As shown in FIG. 1, thefibers were removed by filtering or screening and allowed to dry in airat room temperature and pressure. The fibers were heated in an airatmosphere to 600° C. at 20° C. per minute and maintained in this 600°C. environment for about 30 minutes to assure complete oxidation of thecarbon fiber micro-molds. Six hundred degrees (600° C.) was sufficientto burn off the carbon micro-molds and form transparent alumina fibers,which are shown in FIG. 7 in a SEM photograph at 240 times actual size.

EXAMPLE 3

Several different methods were used to form zirconia fibers via thepiggyback process. These can be separated according to the threedifferent starting materials: Zirconyl Hydroxycloride Trihydrate,supplied by Harshaw Chemical Company, Zirconium Acetate Solution,supplied by Harshaw, and Zirconium n-Butoxide Butanol Complex, suppliedby Alfa Products, Inc.

Where zirconium n-butoxide butanol complex was the starting method, aprecipitate was formed by a process described in Grassi, John Anthony,"Characterization of Parameters Affecting the Properties of anAlkoxid-Derived Zirconia Sol-Gel," Graduate School of ClemsonUniversity, Masters' Thesis, Clemson, S.C., August 1985, which is herebyincorporated herein by reference. The precipitate was dried and calcinedto 1000° C. Calcium carbonate, CaCo₃, and magnesium carbonate, MgCO₃,additions were made to get 12 mol % CaO and 0.1 mol % MgO in the finalproduct. The CaCO₃ was in the form of whiting. The MgCO₃ was aprecipitate. Both passed easily through 400 mesh. The powders weremilled in a plastic jar using alumina grinding media. A slurry was madeby adding water to the powder in a one to one volume ratio. Algin wasadded as a stabilizing agent.

After the suspension was formed, C-shaped carbon fibers were submerged,removed, dried and fired. The firing was done in two ways. In the firstway, the dipped fibers were first fired to 1450° C. in a non-oxidizingatmosphere, then oxidized to burn off the carbon fiber. In the secondway, the fibers were fired in air to 1600° C. X-ray diffraction was usedto determine that the final material was a zirconia fiber in crystalform. There were fibers in the tetragonal phase and some in themonoclinic phase. Zirconia fibers fired in the first way are shown inFIG. 8 in a SEM photograph. The rectangular bar in the lower left cornerof FIG. 8 represents a length of 200 microns for purposes of indicatingthe fiber dimensions. Zirconia fibers fired in the second way are shownin FIG. 9 in a SEM photograph. In FIG. 9a, the firing temperature was600° C., and in FIG. 9b the firing temperature was 1600° C. Therectangular bars in the lower corners of FIGS. 9a and 9b represent alength of 100 microns for purposes of indicating the fiber dimensions.

Where zirconium acetate was used as the starting material for zirconia,a 25% zirconium acetate solution supplied by Harshaw Chemical Companywas used. Either calcium oxide or yttria was employed as the stabilizingagent. Where calcium oxide was the stabilizing agent, 12 mol %equivalent CaO was added by dissolving the correct amount of calciumacetate (Ca(C₂ H₃ O₂).H₂ O) in the zirconium acetate solution. 0.1 mol %MgO was also included with the CaO as a sintering agent. The MgO wasalso added to the zirconium acetate solution in the acetate form (Mg (C₂H₃ O₂)₂. 4H₂ O). Where yttria was the stabilizer, 10 mol % equivalent Y₂O₃ was dissolved into the zirconium acetate solution in the form ofyttrium acetate (Y (C₂ H₃ O₂)₃.xH₂ O, X=2). After the acetate solutionswere prepared, C-shaped carbon fibers were submerged in them, removed,dried, and fired to a temperature as low as 750° C. to form zirconia.X-ray diffraction was used to characterize the fired material aszirconia in the tetrogonal and monoclinic phases. Zirconia fibersproduced from the acetate are shown in FIG. 10 in a SEM photograph inwhich the rectangular bar in the lower left corner represents a lengthof 25 microns for purposes of indicating the fiber dimensions.

Where zirconyl hydroxycloride trihydrate from Harshaw Chemical was theprecursor for zirconia, C-shaped carbon fibers were simply submerged inthe as-received liquid, removed, dried and fired in air. FIG. 11a showsfibers fired to 600° C. in air, and FIG. 11b shows these fibers fired to1600° C. in air. In FIG. 11a, the rectangular bar represents a length of100 microns, and in FIG. 11b the bar represents a length of 200 microns.Zirconia fibers made from this trihydrate precursor appear to be made upof smaller fiber-like particles at 600° C. whereas at 1600° C. theyappear to be more dense an are accompanied by larger non-fiberousparticles. This starting material for the precursor is incapable ofbeing drawn into a fiber or spun into a fiber through a spinerette.

EXAMPLE 4

A dilute HCl solution was added to furfuryl alcohol and mixed well. TEOSwas added to the solution of alcohol and HCl. The TEOS: furfuryl alcoholvolume ratio was 1:1. The H₂ O : TEOS molar ratio was 2:1, and the HCL:TEOS molar ratio was 0.01 : 1.0. This solution was mixed for one daybefore C-shaped carbon fibers were submerged, removed, dried and firedto 1450° C. in an argon atmosphere. After firing in argon, the fiberswere oxidized to 600° C. to remove the carbon fibers and any excesscarbon. The resulting fiber material was characterized by x-raydiffraction as ceramic material such as beta-SiC. These fibers are shownin FIG. 12 in a SEM photograph in which the rectangular bar in the lowerleft corner represents a length of 200 microns for purposes of indictingthe fiber dimensions.

SiC was made from polysilastyrene (PSS). The PSS was produced by amethod similar to that described in R. West et al, "Polysilastyrene:Phenylmethylsilane-Dimethylsilane Copolymers as Precursors to SiliconCarbide," American Chemical Society Bulletin, Vol. 62, No. 8, pp.899-902 (1983), which is hereby incorporated herein by reference, andvia the TEOSIC process® using TEOS and Furfuryl Alcohol supplied byFisher Scientific Co. C-shaped carbon fibers were submerged in aPSS-tolvene solution, removed, dried and pyrolyzed in an argonatmosphere at 1100° C. After pyrolysis, the fibers were oxidized in airat 800° C. for 30 minutes in order to remove the carbon fibermicro-mold. Fourier transform infra-red spectroscopy (hereafter FT-IR)was used to characterize the resulting fibers in terms of the bondswhich were present. This determination established the final fiberproduct to be SiO₂ ceramic fiber. These silica fibers are shown in FIG.13 in a SEM photograph in which the rectangular bar in the upper leftcorner represents a length of 300 microns for purposes of indicating thefiber dimensions.

EXAMPLE 5

A hydroxyapatite slurry (HA-slurry) was supplied by Coors BiomedicalCompany. C-shaped carbon fibers were submerged in HA-slurry, removed anddried. Redipping and drying was repeated two more times. The tripledipped fibers were then placed in a pure nitrogen (N₂) atmosphere whichwas then heated from room temperature to 980° C. at a rate of 4° C. perminute. Then the fibers were removed from the nitrogen atmosphere andplaced in an air atmosphere at room temperature. The fibers then wereheated in the air atmosphere from room temperature to 600° C., at 5° C.per minute. The samples were cooled slowly by leaving them in thefurnace as it cooled after both heat treatments. FIG. 14 shows a SEMphotograph at 50 times magnification for hydroxyapatite ceramic fibersproduced according to the method of Example 5.

EXAMPLE 6

A silica sol was prepared with a 2:1 TEOS to ethanol volume ratio, a 2:1H₂ O to TEOS molar ratio, and a 0.01 : 1.0 HCl to TEOS molar ratio. Thesol was covered with plastic wrap and mixed at a constant rate. Afterthree days, pin-holes were made in the cover to allow some evaporation.After four days the pin-holes were enlarged to about 1/8 inch diameter.After five days the mixing rate was increased by 25%. The viscosity wasmeasured and C-shaped carbon fibers dipped as shown on Table V.

                  TABLE V                                                         ______________________________________                                        Time         Fig.   Viscosity in Centipoise                                   ______________________________________                                         3 days      15a     4.8                                                       6 days      15b     5.8                                                      10 days      15c    15.0                                                      12 days      15d    58.5                                                      ______________________________________                                         1 centipoise (cps) = .01 Poise                                           

As shown in FIG. 15, as viscosity increases, the ceramic fiber portionof the dual fiber produced according to the method of the presentinvention acquires a transverse cross-sectional perimeter that becomesmore circular. Thus, the viscosity of the chemical precursor providessome degree of control over the non-circularity of the transversecross-sectional perimeter of the ceramic fiber portion.

FIG. 16 shows silica ceramic fibers formed by using nylon fibers havinga trilobal transverse cross-sectional perimeter for the non-ceramicmicro-mold portions. The rectangle at the lower left corner of FIG. 16represents a length of 200 microns and indicates the dimensions of thesilica fibers shown therein. FIG. 17 showns the trilobal nylon fiberacting as the micro-mold carrier fiber for the silica fibers shown inFIG. 16. The bar in the corner of FIG. 17 represents a length of 20microns. The silica fibers shown in FIG. 16 were produced according tothe method described in Example 1 above, except that the trilobal nylonfiber was used as the non-ceramic carrier micro-mold fiber instead ofthe C-shaped carbon fiber used in Example 1.

The piggyback method makes possible many kinds of ceramic fibers notpossible by other processes. Many ceramic fibers presently made arelimited by the process of drawing the fibers from a melt and rapidlycooling to prevent devitrification of crystallization. Phase separationcan also be a problem. This is particularly true in the case of glassfibers. Ceramic fibers produced by melt drawing are further limited tocompositions containing appreciable amounts of glass forming oxides suchas SiO₂, B₂ O₅, or P₂ O₅. Also, melt drawing requires high temperaturesand special equipment for the drawing of the fibers. In the piggybackmethod of the present invention, ceramic fibers are formed in a uniqueway which requires neither the use of high temperature melting equipmentnor glass forming oxides as a major constituent of the composition ofthe fiber.

Interest in ceramic-carbon dual fibers has been stimulated by theirpotential uses in composites. Applicant forsees that the ceramiccomponent would act as a crack-eater, thereby toughening a composite andincreasing its damage tolerance. This foresight is based in part on thefact that toughness has been related to fiber pull-out, which is relatedto a low fiber matrix interfacial bond. However, strength of a compositeis related to a strong interfacial bond. By controlling the interfacialbond through control of the composition of the ceramic component of dualfibers, it may be possible to produce a fiber that is strongly bonded tothe matrix on one side, and weakly bonded on the other.

Another potential advantage of incorporating ceramic-carbon dual fibersin a composite may be oxidation resistance. Presently the surface ofcarbon-carbon composites are coated with an oxidation resistant materialso that the composite can be used at higher temperatures. However, abreak in this coating can cause failure of the composite due tooxidation. By incorporating an oxidation resistant ceramic material inthe reinforcing fibers, hence the body of the carbon-carbon composite, asealing mechanism may occur when the ceramic material becomes lessviscous at an elevated temperature. The ceramic component could bedesigned to melt at a given temperature through control of composition.Once fluid, the oxidation resistant ceramic material would be free toflow into pinholes and cracks, protecting against further oxidation.

None of the conventional methods for producing ceramic fiber permit theproduction of ceramic fibers having a non-circular transversecross-sectional perimeter. However, the micro-mold method of the presentinvention makes it possible to produce ceramic fibers havingnon-circular transverse cross-sectional perimeters. As shown in FIG. 5,for example, the use of C-shaped micro-mold carrier fibers yieldsceramic fibers having a generally crescent moon shaped transversecross-sectional perimeter. Similarly, the use of micro-molds having adifferently shaped non-circular cavity would yield a commensuratelyshaped transverse cross-sectional perimeter for the ceramic fiberproduced therewith.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the scope or the spirit of the invention. Thus,it is intended that the present invention cover the modifications andvariations of this invention, provided they come within the scope of theappended claims and their equivalents.

What is claimed:
 1. A piggyback method of forming a ceramic fiber from awet chemical precursor including a metal oxide and being capable ofbeing sintered to form ceramic material, the method consistingessentially of:providing an elongated micro-mold carrier fiber, saidcarrier fiber being formed of a material other than ceramic material andto which said precursor adheres, said elongated carrier fiber definingan elongated cavity therein and over a substantial portion of the lengththereof; wetting the cavity of said carrier fiber with the wet chemicalprecursor; setting the chemical precursor in the cavity of said carrierfiber until the wet chemical precursor is nested in the cavity; placingsaid carrier fiber with the nested chemical precursor into anoxygen-free environment; heating said carrier fiber with the nestedchemical precursor in the oxygen free environment to a temperaturesufficient to sinter the nested chemical precursor into a ceramic fiberand form a dual fiber having a ceramic fiber nested in said carrierfiber; and removing said carrier fiber of said dual fiber to yield saidceramic fiber.
 2. A method as in claim 1 further including the step ofcross-linking the carrier fiber before wetting same in the wet chemicalprecursor.
 3. A method as in claim 1, wherein:the step of removing thecarrier fiber comprises mechanically removing the carrier fiber untilonly the ceramic fiber remains.
 4. A method as in claim 1, wherein:thecarrier fiber is formed of one of the following materials: carbon, apolymeric material.
 5. A method as in claim 2, wherein:the carrier fiberis formed of one of the following materials: carbon, a polymericmaterial.
 6. A method as in claim 1, wherein:the wet chemical precursorconstitutes one chosen from the group consisting of a sol-gel, a slurry,a melt, a suspension, and a dispersion, and includes at least one of thefollowing ceramic materials: silica, alumina, silicon carbide, zirconia,and hydroxyapatite.
 7. A piggyback method of forming a ceramic fiberfrom a wet chemical precursor including a metal oxide and being capableof being sintered to form ceramic material, the method consistingessentially of:providing an elongated micro-mold carrier fiber, saidcarrier fiber being formed of a material other than ceramic material andto which said precursor adheres, said elongated carrier fiber definingan elongated cavity therein and over a substantial portion of the lengththereof; wetting the cavity of said carrier fiber with the wet chemicalprecursor; setting the chemical precursor in the cavity of said carrierfiber until the wet chemical precursor is nested in the cavity; placingsaid carrier fiber with the nested chemical precursor into anoxygen-free environment; heating said carrier fiber with the nestedchemical precursor in the oxygen free environment to a temperaturesufficient to sinter the nested chemical precursor into a ceramic fiberand form a dual fiber having a ceramic fiber nested in said carrierfiber; and removing the carrier fiber of said dual fiber to yield saidceramic fiber by heating the dual fiber in an oxygen-containingenvironment until the carrier fiber is burned away and only the ceramicfiber remains.
 8. A method as in claim 7, wherein:the carrier fiber isformed of one of the following materials: carbon, a polymeric material.9. A piggyback method of forming from a wet chemical precursor, whichincludes a metal oxide and is capable of being sintered to form ceramicmaterial, a ceramic fiber having a non-circular transversecross-sectional perimeter over a substantial portion of the length ofthe fiber, the method consisting essentially of the steps of:providingan elongated micro-mold carrier fiber, said carrier fiber being made ofmaterial other than ceramic material and to which said precursoradheres, said elongated carrier fiber defining an elongated cavitytherein having a non-circular transverse cross-sectional perimeter overa substantial portion of the length thereof; wetting the cavity of saidcarrier fiber in the wet chemical precursor; setting said chemicalprecursor in the cavity of said carrier fiber until the chemicalprecursor is nested in the cavity; placing said carrier fiber with thenested chemical precursor into an oxygen-free environment; heating saidcarrier fiber with the nested chemical precursor in the oxygen-freeenvironment to a temperature sufficient to sinter the nested chemicalprecursor into a ceramic fiber; removing said carrier fiber from saidceramic fiber to yield a ceramic fiber having a non-circular transversecross-sectional perimeter over a substantial portion of the lengththereof.
 10. A method as in claim 9, wherein:the wet chemical precursorconstitutes one chosen from the group consisting of a sol-gel, a slurry,a melt, a suspension, and a dispersion, and includes at least one of thefollowing ceramic materials: silica, alumina, silicon carbide, zirconia,and hydroxyapatite.